This invention relates to opaque and transparent glass-ceramics containing gahnite suitable for use as a substrate material in various electric, electronic, and opto-electronic devices such, for example, as magnetic heads, liquid crystal displays, solar batteries, photomasks and optomagnetic disks. More particularly, the invention relates to two glass-ceramics obtained by melting and heat treating a SiO.sub.2 - Al.sub.2 O.sub.3 - ZnO - MgO - PbO system glass of a specific composition which is substantially free of an alkali ingredient, i.e., an opaque glass-ceramic containing gahnite which is substantially free of flexion in the thermal expansion curve and has an improved chemical durability, and a transparent glass-ceramic containing gahnite having an excellent optical homogeneity, which is obtainable advantageously from the economic standpoint.
There is a growing demand for magnetic heads which are important components for recording, reproducing and erasing information in various devices including audio devices, video tape recorders and computers. Particularly, demands for making these devices more compact and increasing density of information have led to rapid progress in the development of a thin film magnetic head utilizing thin film techniques such as integrated circuits.
The thin film magnetic head is made by forming a thin film of magnetic substance over a substrate. A thin film made of Ni - Zn ferrite, Mn - Zn ferrite, permalloy (Fe - Ni), sendust (Fe - Al - Si) or amorphous substance of such material performs functions of recording, reproducing and erasing information and a substrate performs other functions including supporting the magnetic substance and moving in sliding movement. For this reason, the substrate is required to have the following general properties:
(1) For achieving a good bonding with the magnetic substance, a coefficient of thermal expansion must be selected from a range between about 70-180.times.10.sup.-7 /.degree. C. in accordance with the type of magnetic substance. PA0 (2) The substrate must have about the same hardness as the magnetic substance, i.e., Vickers hardness of about 600-1150 Kgf/mm.sup.2, so as to prevent occurrence of partial wear due to sliding movement of the substrate relative to a magnetic tape or a disk. PA0 (3) The substrate must have a structure which is close, uniform and fine. PA0 (4) The substrate must have a uniform quality and good productivity. PA0 (5) A coefficient of thermal expansion must be selected from a wide range between 25-200.times.10.sup.-7 /.degree. C. PA0 (6) The substrate must have a very fine structure and a transparent and highly uniform article, in addition to an opaque article, must be manufactured therefrom. PA0 (7) The substrate must be substantially free of an alkali ion, since alkali ions in the substrate would be diffused in the thin film with a result that its properties would be degraded. PA0 (8) The substrate must have good refractoriness, i.e., a transformation temperature of about 780.degree. C. or higher, since the substrate is exposed to high temperatures during the-process of forming of the thin film. PA0 (9) The substrate must have sufficient chemical durability against rinsing or etching performed by using various chemical reagents.
In the above described electric, electronic and opto-electronic devices other than magnetic heads also, various components made by bonding or thin-film forming a crystalline or amorphous metal, alloy or metal oxide on substrates are used. These substrates are required to have, in accordance with the material of the components, the following properties:
Glass-ceramics are listed as one of the materials which are poreless and close in structure and therefore are considered to satisfy the above mentioned requirements for substrates. Particularly, there have been disclosed some glass ceramics, as suitable materials for substrates, which are obtained by heat treating a parent glass of the SiO.sub.2 - Al.sub.2 O.sub.3 - ZnO - nucleating agent system and contain crystals of gahnite, willemite, .alpha. - cristobalite or .alpha. - quartz grown therein as the predominant crystal phase.
U.S. Pat. No. 2,920,971, for example, discloses a SiO.sub.2 - Al.sub.2 O.sub.3 - ZnO - TiO.sub.2 system glass as a parent glass in which one or more of Li.sub.2 O, BeO, MgO, CaO, ZnO, SrO, CdO, BaO, PbO and MnO is added to SiO.sub.2, Al.sub.2 O.sub.3 and TiO.sub.2, and discloses also a method for obtaining glass-ceramic articles by subjecting this parent glass to a heat treatment which consists of two stages of heat treating the parent glass for development of nuclei and further heat treating it for subsequent growth of crystals. In the glass-ceramic articles obtained by this method, however, diameters of crystals grown are 0.1-20 .mu. as described in the specification of the U.S. Patent, which are too coarse to meet the above described requirement (6) concerning the fineness and transparency of the structure of the substrate. Besides, a glass-ceramic article among the articles in this U.S. Patent which is obtained from a parent glass containing a relatively large amount of ZnO tends to be accompanied by growing of willemite crystals and such article containing willemite crystals has extremely poor chemical durability. Further, in a case where crystals which are of high thermal expansion characteristics are to be grown for obtaining a glass-ceramic article of relatively high thermal expansion characteristics, .alpha.-cristobalite crystals which cause a considerable flexion due to phase transformation in the thermal expansion curve of the article are grown excessively which adversely affects bonding of the article with a magnetic or other substance.
U.S. Pat. No. 3,681,102 discloses transparent glass-ceramics obtained by subjecting a SiO.sub.2 - Al.sub.2 O.sub.3 - ZnO - ZrO.sub.2 system parent glass to the two-stage heat treatment and containing gahnite as the predominant crystal phase. The glass-ceramics have fine diameters of crystals which are less than 1000 .ANG. (0.1 .mu.). Since, however, the melting temperature of the parent glass is a high one in the range of 1650.degree.-1800.degree. C., it is not easy to obtain an optically uniform glass-ceramic article.
For improving the transparency of the finished articles and the melting property of the parent glass of these transparent glass-ceramics which have been found inadequate, U.S. Pat. No. 4,687,750 discloses transparent glass-ceramics obtained by applying the two-stage heat treatment to a parent glass in which the SiO.sub.2 - Al.sub.2 O.sub.3 - ZnO sytem glass and a nucleating agent of TiO.sub.2 and (K.sub.2 O+Rb.sub.2 O+Cs.sub.2 O) coexist. In these glass-ceramics, the fineness of diameters of crystals has been improved to less than 300 .ANG.. The melting property of the parent glass, however, is not considered to have been sufficiently improved in these glass-ceramics. Indeed, as described in the specification of this U.S. Patent, it requires melting of the parent glass at about 1600.degree. C. for 16 hours in a platinum crucible to obtain a slab of about 6".times.6".times.0.5". This results in increase in cost of the article and therefore is not advantageous economically. This also makes it difficult to obtain an article having an excellent homogeneity. Moreover, an alkali ingredient must essentially be contained in these glass-ceramics.
Japanese Patent Publication No. 46-16518 discloses a fortified glass-ceramic obtained by applying a one-stage stage heat treatment to a SiO.sub.2 - Al.sub.2 O.sub.3 - ZnO - MgO - ZrO.sub.2 system glass to have .beta. -quartz which is of low thermal expansion characteristics crystallized in its surface layer and .alpha. -quartz or spinel which is of higher thermal expansion characteristics crystallized in its interior portion. Japanese Laid-open Patent Publication No. 59-203736 discloses a high thermal expansion glass-ceramic obtained by heat treating a SiO.sub.2 - Al.sub.2 O.sub.3 - ZnO - ZrO.sub.2 - TiO.sub.2 system glass and containing .alpha. -quartz and .alpha. - critobalite as the prodominant crystal phases. These glass-ceramics, however, have considerable flexions in their thermal expansion curves. Besides, it is difficult in these glass-ceramics to prevent the tendency to increasing coarseness of crystal grains despite attempted efforts in improving the heat treatment conditions.